1. Field of the Invention
The present invention relates to an image pickup apparatus having an iris member and filter units.
2. Related Background Art
The following is an example of a conventional technology in this category of field.
FIG. 8 is a block diagram showing a configuration of a conventional image pickup apparatus.
Referring to FIG. 8, there are shown a lens 1 for forming an image of an object, an iris mechanism 2 for controlling an amount of incident light, an iris driving motor 3 for driving (moving) the iris mechanism 2, an iris mechanism drive device for driving the iris driving motor 3, an iris state detecting device for detecting a state of the iris mechanism 2, an ND filter 6 for controlling the amount of the incident light, an ND driving motor 7 for driving the ND filter 6, an ND mechanism drive device 8 for driving the ND driving motor 7, and an ND state detection device 9 for detecting a state of the ND mechanism (filter) unit 6.
There are further shown a CCD (Charge-Coupled Device) 10 for performing a photoelectric conversion of the incident light, an image pickup element drive device 11 for controlling the CCD 10 to read photoelectrically converted signals and controlling a signal accumulation time, i.e., controlling a so-called electronic shutter function, a CDS/AGC 12 for sampling the photoelectrically converted signals and electrically amplifying the signals, an analog-to-digital converter (which will be hereinafter be abbreviated to an A/D converter) 13 for converting an analog signal as an output of the CDS/AGC 12 into a digital signal, a signal processing device (which will hereinafter be called a DSP) 14 having a control function of executing processing such as a color separation, a color difference matrix, etc. after a gamma correction and thereafter generating standard TV signals by adding synchronous signals, a memory 15 for storing an image processed by DSP 14, a recording medium 16 capable of recording still images and image information thereof, a microcomputer 17 for issuing processing commands to the DSP 14, and a liquid crystal panel 18 for displaying the image.
Next, a method of controlling the iris mechanism 2 and the ND filter 6 will be discussed.
The light incident on the lens 1 travels through the iris mechanism 2 and the ND filter 6, and the light restricted by these optical elements enters the CCD 10. Signals photoelectrically converted by the CCD 10 are converted into the digital signals by the A/D converter 13 cooperating with the CDS/AGC 12 and are objected to camera-signal-processing in the DSP 14. The DSP 14 transmits luminance data corresponding to an exposure control frame to the microcomputer 17, wherein a calculation for the exposure control is made based on the luminance data. If a result of this calculation is not proper to the exposure, the iris mechanism 2, the ND filter 6, the electronic shutter and the AGC are controlled so that the calculation result becomes proper.
Among these four exposure control parameters, a control relationship between the iris mechanism 2 and the ND filter 6 will be explained. To start with, when controlling the exposure in such a direction as to reduce the incident light from a state where the iris mechanism 2 remains open and the ND filter 6 entirely withdraws from an aperture diameter of the iris mechanism 2, the iris mechanism 2 closes up (stops down) to a certain aperture diameter, and, after this aperture diameter of the iris mechanism 2 has been fixed, the ND filter 6 is consecutively advanced stepwise, thereby controlling the exposure with the ND filter 6 (see FIGS. 9A to 9E).
What has been required of the exposure control of the conventional image pickup apparatus is given as follows. This will be explained referring to FIGS. 9A to 9E.
Under the conventional exposure control, as shown in FIG. 9B, the ND filter 6 is advanced halfway such as only by a half-insertion towards the aperture of the iris mechanism and, as shown in FIGS. 9C and 9D, different-density areas of the ND filter 6 dually cover the aperture diameter of the iris mechanism 2.
As described above, if the ND filter 6 is advanced halfway towards the aperture diameter of the iris mechanism 2 or the different-density areas of the ND filter 6 cover the aperture diameter, there arises a problem in which an image resolution declines due to an occurrence of diffraction of the light that is caused by level differences both in thickness and in density of the ND filter 6. Therefore, the ND filter 6 cannot be inserted for still image photography that gives the priority to the image resolution so that a dynamic range of the exposure control decreases to an extent corresponding to a density of the ND filter 6 as compared with the moving image photography.